GB2173055A - Circuit arrangement for starting discharge lamps - Google Patents

Abstract

During an initial part of a starting cycle, a semiconductor switching device 24 presents a low impedance path between electrodes 15, 16 which therefore heat up until a capacitor 37 has charged sufficiently to turn on SCR 32 causing device 24 to turn off and a high voltage ignition pulse to be generated with the aid of inductive ballast 12. Means are provided to effectively isolate the capacitor 37 from the A.C. supply voltage during running of the lamp 14 so that the capacitor 37 is prevented from being maintained charged and thus SCR 32 is prevented from being held conductive. The isolating means allows capacitor 37 to discharge via path 38, 39, 26, 27 when the lamp is running so that the lamp can be restarted if the A.C. supply is interrupted even for a relatively short period. The isolating means may be a bidirectional breakover device 40 located at the input to a rectifier bridge 18, 19, 20, 21 and having a breakover voltage greater than the operating voltage of the lamp 14. Alternatively, a unidirectional breakover device may be located at the output of the rectifier bridge, or bridge diodes 18, 19 may be breakover diodes, or a resistor 29 may be a voltage dependent resistor which presents a very high resistance when the lamp is running. <IMAGE>

Description

SPECIFICATION Circuit arrangement for starting discharge lamps This invention relates to a circuit arrangement for starting an AC energised gas and/or vapour discharge lamp having preheatable electrodes and provided with an inductive ballast, the circuit arrangement being adapted for connection between the preheatable electrodes of the lamp and having a semiconductor switching device switchable between a first state in which it operates to present a low impedance path between the electrodes during an initial part of the starting cycle to preheat the electrodes and a second state in which it operates to present a high impedance path between the electrodes after a delay so as to cause a high voltage pulse to be applied across the lamp electrodes by the inductive ballast for striking the lamp, the semiconductor switching device being switched from its first to its second state via a capacitor in dependence upon charge stored in the capacitor during electrode preheating by means of capacitor charging circuit, the capacitor being provided also with a discharge path. The invention further relates to a lamp unit including such a circuit arrangement.

Such an electronic starter circuit arrangement may be used in place of the common electro-mechanical glow lamp type starter circuit and performs a similar function to allow heating current to flow through the lamp electrodes for a short period of time and then, in effect, to interrupt the circuit, whereupon the inductive ballast generates a momentary high voltage by self induction which is applied to the lamp electrodes in order to strike the lamp. When struck the voltage across the running lamp decreases to a comparatively low value. The electronic starter circuit arrangement has advantages over the glow lamp type starter circuit. With a glow lamp type starter circuit, interruption of the circuit more often than not occurs before the lamp electrodes have reached the required lamp-striking temperature so that repeated attempts at striking the lamp are necessary.Moreover, since the point in time at which the starter circuit is interrupted is random to some extent, the current flowing in the circuit when interruption actually occurs could be low or even zero in which case insufficient induction voltage will be available for successfully striking the lamp and the starter circuit will repeat its attempts thereby causing undesirable flashes. With the electronic starter circuit arrangement however, the preheating period and the point at which the circuit is effectively interrupted, that is, the point at which the low impedance presented to the electrodes is switched to high impedance, can be accurately defined, thereby ensuring that succesful ignition of the lamp is achieved quickly.

An example of such an electronic starter circuit arrangement is that supplied by Arlen Electrical plc under their reference code EFS 500 and which is intended to be used as a direct replacement for a glow lamp type starter circuit. In this known arrangement, the semiconductor switching device is an integrated power function device comprising a thyristor requiring a high holding current. Such a switching device is described in greater detail in published European Patent Application 0118309 which also describes examples of electronic starter circuits employing the device similar in respects to the aforementioned starter circuit arrangement from Arlen Electrical plc.

The semiconductor switching device is arranged so that during an initial phase of the starting procedure, it presents in its first state a low impedance path between the lamp electrodes, effectively short-circuiting the electrodes, whereupon current flows from an AC source through the electrodes to preheat them. During this stage, the capacitor in the circuit arrangement is charged up through its charging circuit which comprises a resistive network. When sufficiently charged, the time taken to reach this point corresponding roughly to that needed for achieving the desired preheating of the lamp electrodes, the capacitor triggers a further thyristor which causes the device to switch to its second, non-conducting state and, as a result, a high impedance to be presented across the lamp electrodes, thereby producing a striking voltage across the lamp.If the lamp fails to strike, the cycle is repeated to produce, if necessary, a succession of striking pulses, the intervals between successive pulses decreasing due to a gradual increase each time if the initial charge in the capacitor through its inability to discharge quickly via its associated discharge circuit.

Whilst the circuit operates generally satisfactorily, it has been found that in certain circumstances a disadvantage may arise. More particularly it has been observed that if, during running of the lamp a short interruption in the AC supply to the lamp occurs, the starter circuit arrangement may not operate to restrike the lamp following resumption of power, and so the lamp can remain off indefinitely. It is believed that this phenomena may be due to the fact that the capacitor stores a charge during running of the lamp so that, if the power interruption is less than the time required for the capacitor to discharge sufficiently through its associated discharge circuit as determined by its time constant, the capacitor acts to trigger the further thyristor thus preventing correct operation of the semiconductor switch device to produce a striking pulse.In order for the lamp to be restruck, it is necessary to disconnect the AC supply for a period of time sufficient to allow the capacitor to discharge at least to a level at which it does not trigger the further thyristor.

It is an object of the invention to provide a circuit arrangement of this kind but which does not suffer from the aforementioned disadvantage.

According to one aspect of the invention there is provided a circuit arrangement for starting an AC energised gas and/or vapour discharge lamp having preheatable electrodes and provided with an inductive ballast, the circuit arrangement being adapted for connection between the preheatable electrodes of the lamp and having a semiconductor switching device switchable between a first state in which it operates to present a low impedance path between the electrodes during an initial part of the starting cycle to preheat the electrodes and a second state in which it operates to present a high impedance path between the electrodes after a delay so as to cause a high voltage pulse to be applied across the lamp electrodes by the inductive ballast for striking the lamp, the semiconductor switching device being switched from its first to its second state via a capacitor in dependence upon charge stored in the capacitor during electrode preheating by means of capacitor charging circuit, the capacitor being provided also with a discharge path wherein the circuit arrangement further includes semiconductor means connected in series with the semiconductor switching device and capacitor charging circuit between the lamp electrodes and which, in use, allows current to flow therethrough during starting, but which operates to prevent substantially any current flow therethough whilst the lamp is running.

By providing the semiconductor means operative in this manner, the capacitor is effectively isolated from the AC supply voltage during running of the lamp and is therefore able to discharge completely through its associated discharge circuit. Hence, in the event thereafter of a short interruption of the AC supply, the circuit arrangement can, upon resumption of the AC supply, operate in its intended way to restrike the lamp. Thus, the requirement to disconnect the AC supply for any time is avoided.

The semiconductor means preferably is responsive to a predetermined level of voltage to switch from a substantially non-conducting state to a conducting state and is arranged in the circuit arrangement so as, in use, to conduct in response to voltages existing in the circuit arrangement during starting and is substantially non-conducting whilst lower voltages exist during running of the lamp. In this way therefore, the semiconductor means advantageously utilises the operational characteristics of the lamp, and more precisely the drop in voltage across the lamp which occurs when the lamp is ignited, and responds automatically so as to enable the semiconductor switching device and capacitor of the circuit arrangement to operate as required during lamp starting and to isolate those components electrically whilst the lamp is running.

The semiconductor means may comprise one or more voltage responsive breakdown devices, for example, break over diode (BOD) devices such as Mullard BODs or similar devices such as Motorola Sidac devices. The circuit arrangement may further include a full wave diode bridge rectifier whose AC input is to be connected across the lamp electrodes and whose DC output is connected to the semiconductor switching device and capacitor charging circuit. In this case, the voltage responsive breakdown device may for simplicity be a bidirectional device, for example a bidirectional BOD, capable of responding to either voltage polarity so as to conduct in both directions and connected in the AC input of the diode bridge rectifier.As such, the semiconductor means is constituted by a single component which is easily and conveniently connected in the circuit arrangement, thus making modification of the aforementioned known circuit arrangement relatively simple. It can be of small size and so does not add significantly to the overall bulk of the circuit arrangement.

Alternatively, two unidirectional breakover devices responsive to only one voltage polarity so as to conduct in one direction may be connected together with two diodes so as to constitute the full wave rectifier.

According to another aspect of the present invention, there is provided a lamp unit comprising a gas and/or vapour discharge lamp having preheatable electrodes, an inductive ballast connected in series with the lamp between AC supply terminals, and a circuit arrangement in accordance with said one aspect of the invention connected across the preheatable electrodes.

A circuit arrangement, and lamp unit including the circuit arrangement, in accordance with the invention, will now be described by way of example with reference to the accompanying drawing which shows the circuit arrangement together with a discharge lamp and asociated inductive ballast impedance.

Referring to the drawing, AC supply input terminals are designated 10 and 11. These terminals, which are intended to be connected to an alternating current, 50Hz, supply mains, are shunted by a series arrangement of an inductive ballast 12 and a low-pressure, mercury vapour discharge lamp 14 having preheatable electrode 15 and 16. In this embodiment the inductive ballast 12 is a coil, but may in certain cases alternatively be formed by a leakage transformer.The leads of the electrodes 15 and 16 of the discharge lamp remote from the input terminals 10 and 11 are connected together through a starting circuit arrangement which, upon the supply of AC mains voltage to the terminals 10 and 11, operates to cause the electrodes 15 and 16 of the lamp to be heated to a required temperature during an initial part of the starting cycle and then after this preheating delay, to cause, in conjunction with the ballast 12, a series of high voltage striking pulses to be applied across the lamp to ignite the lamp.

The starting circuit arrangement, shown to the right of the lamp 14 in the drawing, includes a full wave diode bridge rectifier comprising four diodes 18, 19, 20, 21 whose AC input is connected to the leads of the lamp electrodes 15 and 16 remote from the terminals 10 and 11, and whose DC output is connected to points 22 (positive) and 23 (negative).

The points 22 and 23 are interconnected through three parallel circuits. In one of these parallel circuits, the point 22 is connected to the anode terminal of power semiconductor switching device 24 acting as a thyristor which requires a high holding current and being switchable between a first state in which it conducts between its anode and cathode and a second, non-conductive, state. More particularly, the switching device comprises a "Fluoractor" device made by Texas Instruments. Details of the construction and operation of this device and its use in lamp starting circuits are given in published European Patent Application No. 0118309. Briefly, however, the device is a monolithic power semiconductor structure which includes a primary thyristor and a secondary thyristor whose anodes are coupled together.The cathode of the secondary thyristor is connected to the gate of the primary thyristor and also through a resistance to the cathode of the primary thyristor, this cathode also constituting the output cathode of the device. The gate of the secondary thyristor acts as the gate of the device and is connected through a resistance to its cathode. The structure of the primary thyristor is such that it requires a high holding current to keep it conducting when positive bias on the gate is not present. Also included in the device structure is a zener diode whose anode and cathode are connected respectively to the cathode and anode of the primary thyristor. Whilst the device thus comprises a number of interconnected components, it has been represented in the drawing as a thyristor for simplicity.The cathode of the device 24 is connected to point 23 through three series connected diodes 25, 26 and 27.

The second of the parallel circuits comprises two series connected resistors 29 and 30, the resistor 29 shunting the device 24 and diode 25, and the resistor 30 shunting the diodes 26 and 27.

The third parallel circuit comprises a resistor 31 in series with a further thyristor 32.

The gate of switching device 24 is connected to the junction between resistor 31 and the anode of thyristor 32. The gate of thyristor 32 is connected to the junctions between diodes 25 and 26 and resistors 29 and 30 through a resistor 33, and to the point 23 through a resistor 34. The gate of thyristor 32 is also connected through a series circuit comprising a resisitor 36 and capacitor 37 to point 23. A series combination of diode 38 and resistor 39 connects the junction between capacitor 37 and resistor 34 to the junctions between diodes 25 and 26 and resistors 29 and 30. The diode 38, resistor 39 and diodes 26 and 27 together constitute a series discharge circuit for the capacitor 37.

The circuit arrangement described thus far is similar to that of the Arlen Electrical plc. 's EFS 500 product. Its operation is as follows. Upon mains voltage initially being supplied to terminals 10 and 11, a full-wave rectified voltage is applied, via the diode bridge circuit, to points 22 and 23. Electrical current flowing through the resistor 31, the device 24 via its gate, diode 25 and resistor 30 triggers the device 24 into conduction so as to present across to the lamp electrodes a low impedance current path through the device 24 and diodes 25 to 27. As a result, a high current flows in both half cycles of the AC supply through the electrodes 15 and 16 which are heated thereby.

The ballast 12 and electrodes 15 and 16 offer a certain impedance to control the current.

During this phase of the starting procedure, current flowing through the device 24 establishes a varying DC voltage (100 Hz) across the diodes 26 and 27 which charges up capacitor 37 via resistors 33 and 36, the components serving as a charging circuit for the capacitor 37. The voltage at the gate of thyristor 32 is therefore increased in accordance with charge in capacitor 37 and, after a while, corresponding approximately in time to that needed for the lamp electrodes 15 to 16 to reach their required temperature and determined accordingly by appropriate choice of component values, the thyristor 32 is triggered into conduction thereby dropping the voltage at the gate of the device 24.Since the current through the device is varying (100Hz), the device 24 switches to its nonconductive state and ceases conduction the next time that current falls below the high holding current necessary to maintain conduction. In so doing, the low impedance path disappears and instead a high impedance is presented across the lamp electrodes 15 and 16.

The energy which has been stored in the inductive ballast 12 is then applied as a high voltage pulse superimposed on the mains voltage waveform across the electrodes 15 and 16 to strike the lamp.

The thyristor 32 ceases conduction when the varying voltage across points 22 and 23 falls to zero and is prevented from turning on again as the voltage rises because the voltage at its gate is too low. The cycle is then re peated if necessary with the capacitor 37 being recharged and again triggering the thyristor 32. Because, however, there is some residual charge in the capacitor 37 at the commencement of this next cycle, the thyristor 32 is triggered slightly earlier. If the striking pulse fails to ignite the lamp, the cycle is repeated, with the thyristor 32 being triggered progressively earlier each time, until eventually voltage at the gate of thyristor 32 is such that the thyristor is held permanently conducting. In this event, it is necessary to interrupt the supply voltage for a while.Once ignition is accomplished and the lamp is running the voltage across the lamp, equal to the discharge voltage, drops significantly below the mains supply voltage.

It has been found that with the starting circuit thus far described, an undesirable phenomenum can occur in use in that if when the lamp is running the mains supply is interrupted for a relatively short interval, the circuit arrangement can fail to re-ignite the lamp upon resumption of power. It is believed that this may be caused by the capacitor 37 being charged to such an extent during running of the lamp through other circuit components, namely resistors 29, 33 and 36, that the thyristor 32 is continuously triggered. Charge in the capacitor 37 is dissipitated through the discharge circuit comprising diode 38, resistor 39 and diodes 26 and 27.However the time constant of this discharge circuit may be in the order of a several seconds so that, in the event of AC power being interrupted for a few seconds only, or less, the capacitor 37 may not be allowed to discharge sufficiently, and, upon resumption of power, could act to hold thyristor 32 conducting thereby preventing proper operation of the circuit arrangement to re-start the lamp.

In order to avoid this problem the circuit arrangement, in accordance with the invention, further includes semiconductor means 40 connected in series between the lead from one of the lamp electrodes, in this case electrode 15, and the associated input of the diode bridge rectifier, and thus in series with the switching device 24 of the charging circuit of the capacitor 37 between the lamp electrodes. The semiconductor means 40 operates to allow current to flow therethrough and therefore electrically connect that input to the electrode lead during lamp starting and to prevent substantially any current flow therethrough, thus electrically isolating that input, during running of the lamp.In this way, the capacitor 37 in the circuit arrangement is allowed to discharge whilst the lamp is running so that in the event of even a very brief mains supply interruption the circuit arrangement can operate immediately upon the resumption of power to restart the lamp in the above described manner.

The semiconductor means 40 is a voltage responsive breakdown device, and more particularly a break over diode device for example a Mullard BR210 BOD device may be suitable.

This device can be described in effect as a kind of triac (without a gate control terminal) which switches on automatically to conduct between its terminals as it reaches a certain predetermined voltage level. The principle of its operation is that in the normal steady state conditions where the voltage across the device is less than that certain voltage, the device adopts a quiescent state in which it blocks any significant current flow therethrough. (A very small amount of leakage current may be passed but this is usually insignificant.) As soon as a certain voltage appears across the device, the "breakover voltage", the device switches on rapidly to a very low voltage (i.e. the voltage drop across the device) so as to conduct and remains in this condition until the current therethrough falls below its holding current, at which time the device automatically switches back to its quiescent blocking state. The device 40 has symmetrical characteristics so that it is bi-directional and can respond to voltages of either polarity to conduct in both directions. The "stand-off voltage", that is, the maximum voltagewhich the device is required to sustain without ever switching on during steady conditions is typically around 85% of the breakover voltage.

In the circuit arrangement, the device 40 has a breakover voltage of less than the peak of the AC mains supply voltage but greater than the discharge voltage across the lamp when it is running. During the starting procedure therefore the device conducts for approximately 145 out of each half mains cycle, since it switches on to a low voltage like a triac in either quadrant, and allows the circuit arrangement to operate as described to start the lamp. When the lamp ignites, the lamp voltage drops below the device's breakover voltage so that the device remains in its quiescent, substantially non-conducting, state, thereby in effect isolating electrically the remainder of the circuit arrangement and ensuring that no charging of the capacitor 37 occurs during running of the lamp. Residual charge on the capacitor 37 is then dissipated through its discharging circuit.

In an alternative embodiment, and instead of using a bidirectional break over diode device as described above, a pair of unidirectional breakover diodes devices responsive to only one voltage polarity to conduct in one direction may be used in place of two of the diodes, 18 and 19, in the diode bridge rectifier.

The two unidirectional break over diode devices serve in effect the same function as the device 40 as well as constituting two diodes of the full wave diode bridge rectifier. Alternatively a single unidirectional break over device may be used in one DC output of the bridge to achieve the same ends.

Whilst with regard to the above circuit arrangements breakover diode devices have been described, it will be appreciated that other kinds of semiconductor means performing a similar function may alternatively be employed. The semiconductor means may comprise a zener device. Alternatively, the resistor 29 may comprise a voltage dependent resistor which presents a very high resistance to the low voltage existing across the lamp during running of the lamp.

Claims (9)

1. A circuit arrangement for starting an AC energised gas and/or vapour discharge lamp having preheatable electrodes and provided with an inductive ballast, the circuit arrangement being adapted for connection between the preheatable electrodes of the lamp and having a semiconductor switching device switchable between a first state in which it operates to present a low impedance path between the electrodes during an initial part of the starting cycle to preheat the electrodes and a second state in which it operates to present a high impedance path between the electrodes after a delay so as to cause a high voltage pulse to be applied across the lamp electrodes by the inductive ballast for striking the lamp, the semiconductor switching device being switched from its first to its second state via a capacitor in dependence upon charge stored in the capacitor during electrode preheating by means of capacitor charging circuit, the capacitor being provided also with a discharge path, wherein the circuit arrangement further includes semiconductor means connected in series with the semiconductor switching device and capacitor charging circuit between the lamp electrodes and which, in use, allows current to flow therethrough during starting, but which operates to prevent substantially any current flow therethough whilst the lamp is running.

2. A circuit arrangement according to Claim 1, wherein the semiconductor means is responsive to a predetermined level of voltage to switch from a substantially non-conductive to a conductive state and is arranged in the circuit arrangement so as, in use, to conduct in response to voltages existing in the circuit arrangement during starting of the lamp and is substantially non-conducting whilst lower voltages exist during running of the lamp.

3. A circuit arrangement according to Claim 2, wherein the semiconductor means comprises one or more voltage responsive breakdown devices.

4. A circuit arrangement according to Claim 3, wherein the or each breakdown device is a break over diode device.

5. A circuit arrangement according to Claim 3 or Claim 4, the arrangement further including a diode bridge rectifier whose AC input is to be connected across the lamp electrodes and whose DC output is connected to the semiconductor switching device and capacitor charging circuit, wherein the voltage responsive breakdown device is connected in the AC input of the diode bridge rectifier, the device being a bidirectional device capable of responding to either voltage polarity so as to conduct in both directions.

6. A circuit arrangement according to Claim 3 or Claim 4, the arrangement further including a full wave rectifier whose AC input is to be connected across the lamp electrodes and whose DC output is connected to the semiconductor switching device and capacitor charging circuit, wherein the semiconductor means comprises two unidirectional voltage breakdown devices responsive to only one voltage polarity so as to conduct in one direction, the unidirectional voltage breakdown devices being connected with two diodes so as to constitute the full wave rectifier.

7. A circuit arrangement substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.

8. A lamp unit comprising a gas and/or vapour discharge lamp having preheatable electrodes, an inductive ballast connected in series with the lamp between AC supply terminals, and a circuit arrangement according to any one of Claims 1 to 7 connected across the preheatable electrodes.

9. A lamp unit substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.